In recent years, an optical element which attempts to realize a polarization optical characteristic using a three-dimensional structure has been developed. Such an optical element, which has a three-dimensional structural body, is disclosed in, for example, U.S. Published Application No. 2007-0201135. A hollow structure is formed in a layer to realize a low refractive index which is not present in a bulk. Furthermore, anisotropy is obtained in an in-plane direction as in a line-and-space structure, so that a high polarization characteristic is realized.
Such a hollow structure is formed using a photolithography process and/or a dry etching technology.
Furthermore, in the case of obtaining a structure which is optically transparent, a dielectric material is generally used as a structural material.
Furthermore, in order to obtain a three-dimensional structure stacked, a space is filled with a material, which is called a sacrificial layer and can be removed in a subsequent step, and a planarizing process is performed. Thereafter, a second layer is deposited and patterned using a photolithography process and/or a dry etching technology.
Then, the sacrificial layer is subjected to ashing from the space of the second layer, and a part in which the sacrificial layer of a first layer has been filled is used as a hollow structure.
In the case where an optical element with a three-dimensional structural body is produced using the above-described conventional method, the first and the second layers may be made of dielectric materials. In this case, if the second layer is not accurately etched using, for example, a time control method, the first layer may be etched, that is, over-etching may occur, or the second layer may not be etched out and partially remain, that is, under-etching may occur.
Furthermore, even though accurate etching is performed, if a large-size base of 6 inches or 8 inches is used as a process base, a process may not be performed uniformly over the whole base surface.
In addition, when the second layer is over-etched and so the first layer is also etched, a part which is extremely small in refractive index is generated. Therefore, it becomes difficult to obtain desired optical characteristics.
Furthermore, when the second layer is under-etched, it is difficult to ash the sacrificial layer which remains in the space of the first layer.
For the forgoing reasons, a hollow structure can not be formed in the first layer, and the refractive index becomes higher. It is also difficult to obtain desired optical characteristics.
In addition, when ions collide with a sidewall, side-etching is performed. As a result, an area of a part which is coupled with a second structural body becomes smaller, and the bonding strength between a first structural body and the second structural body lowers.